HomeMy WebLinkAboutCT 02-19; BRESSI RANCH PA 11; GEOTECHNICAL UPDATE LETTER; 2010-02-10Leighton and Associates, Inc.
A LEIGHTON GROUP COMPANY
February 10, 2010
Project No. 042458-001
To: Manolia Partners, LLC
1094 Cudahy, Suite 212
San Diego, California 92110
Attention: Mr. John Colucci
Subject: Geotechnical Update Letter, Magnolia Estates, Bressi Ranch, PA- i 1, California
References: California Building Standards Commission .(CBSC), 2007, California Building
Code (CBC)
Leighton, and Associates, 2007, Geotechnical Preliminary Investigation for Mass
Grading, Bressi Ranch Carlsbad, California, Project No. 971009-0005, dated
March 14, 2001
Leighton and Associates, 2004, As-Graded Report of Rough and Fine Grading,
Lots 1 through 25, Planning Area PA-i1, Bressi Ranch, Carlsbad Tract No. 00-
06, Carlsbad, California, Project No. 971009-014, dated October 8, 2004
In accordance with your request, this letter has been prepared to update the geotechnical
conditions of Lots 6 through 19, Magnolia Estates located in: Bressi Ranch, Carlsbad, California.
The purpose of our geotechnical update evaluation was to observe the existing geotechnical
conditions of the subject site, review the project geotechnical reports, and update geotechnical
recommendations for the seismic design parameters per• 2007 California Building Code, and
provide appropriate conclusions and additional recommendations, if necessary, for the proposed
development of the site.
Based on our recent site reconnaissance on February 9, 2010, and review ofthe referenced
project geotechnical report,.the geotechnical conditions of the subject site have not significantly
changed since the completion of grading in 2004 (Leighton, 2004), excluding the growth of
weeds, minor desiccation and slightly weathered near surface soils. Currently, Lots 8 through 10
are completed and landscaped, lots 14 through 19 are at various stages of construction, and lots
3934 Murphy Canyon Road, Suite B205 • San Diego, CA 92123-4425
• 858.292.8030 • Fax 858.292.0771
- -001 042458
6, 7, and 11 through 13 are fine graded lots. Therefore, it is our professional opinion that the
geotechnical recommendation presented in the As-Graded Report of Rough and Fine Grading are
still applicable for its intended use, provided the following recommendations are incorporated
into the design of structures, the site grading and construction of the buildings/structures.
In addition, we recommend that dry weathered finish grade soils on lots 6, 7, and 11 through 13 be
re-processed (i.e., scarified a minimum of 12 inches, moisture-conditioned to optimum moisture-
content) and recompacted to a minimum 90 percent relative compaction (based on ASTM Test
Method D1557). Prior to grading, the area should be stripped of surface vegetation and debris.
Existing underground utilities should be anticipated during grading. It should be noted that the
backfill associated with utility trenches on Lot 6 are considered undocumented fill and will require
complete removal prior to placement of additional fill or construction of foundations. Also, note
that the soil on proposed building pads 6, 7, and 11 through 13 were previously tested for
expansion potential and determined to be a Medium with an Expansion Index, and potential for
soluble sulfate attack of the finish grade soils on the building pad were tested and found to possess
moderate to severe soluble sulfate contents (Leighton, 2004). In addition, Lots 14 through 19
needs some minor grading so there is positive drainage of surface water away from structures.
Updated Seismic Recommendations
We have provided updated seismic design parameters per California Building Code (CBC 2007)
for proposed Magnolia Estates. The following seismic design parameters have been determined
in accordance with the 2007 CBC and the USGS Ground Motion Parameter Calculator (Version
5.0.9a).
CBC (2007) Seismic Code - Parameters for the Site
Description Values J CBC Reference
Site Class D Table 1613.5.2
Short Period Spectral Acceleration Ss 1.123 Figure 16 13.5(3)
1-Second Period Spectral Acceleration S1 0.425 Figure 16 13.5(4)
Short Period Site Coefficient Fa 1.051 Table 1613.5.3(1)
1-Second Period Site Coefficient F 1.575 Table 16 13.5.3(2)
Adjusted Short Period Spectral Acceleration 5MS 1.181 Equation 16-37
Adjusted 1-Second Period Acceleration 5M1 0.669 Equation 16-38
Design Short Period Spectral Response Parameter SDS 0.787 Equation 16-39
Design 1-Second Period Spectral Response Parameter SDI 0.446 Equation 16-40
-2- 4
Leighton
042458-001
The recommendations provided in this letter are based on as-graded subsurface conditions
observed 'during the previous site grading. The interpolated subsurface conditions should be
checked in-the field during construction: Construction observation of all onsite excavations and
field density testing -of all compacted fill should be performed by a representative of this office.
All footing excavations should be reviewed by this office prior to placing steel or concrete.
If you have any questions regarding this letter, please contact this office. We appreciate this
opportunity to be 'of service.
Respectfully submitted, oESSio4,
wGHToN AND
ASSOCIATES,
A
NO. 2457 C Vit-
ENGINEERING
Mike D. Jensen, CEG 2457 EXP. William D. Olson, RCE 45283
Project Geologist
'
Associate Engineer
Distribution: (2) Addressee
-3-
4-
- -
Leighton